Add support for NET4.0 to the nuget package

[lvv83]

Hello.
Entity framework 6.x nuget package support for NET4.0 and NET 4.5, but EFInitializer has NET 4.5 only.
We can't use it in NET 4.0 project without reference upgrade.

I think, that your initializer will work fine under NET 4.0 framework, so net45 subfolder is useless.

[lvv83]

Please check my pull request #8